This disclosure relates to a pulse starting method and circuit to pulse the primary winding of a high voltage transformer used to start a gas discharge (e.g. High Intensity Discharge (HID)) lamp. A gas discharge lamp typically uses a ballast circuit to convert an AC line voltage to a Low frequency bi-directional voltage. The ballast circuit includes a converter to convert the AC line voltage to a DC voltage and an inverter which converts the DC voltage to a Low frequency bi-directional voltage. The inverter can take the form of a series half-bridge or full bridge type connected to a DC voltage bus. In addition, a pulse starting circuit can be provided to cold start the gas discharge lamp.
One method and circuit to of igniting an HID lamp is a circuit as illustrated in FIG. 3. As illustrated in FIG. 4, this circuit provides a high voltage pulse 50 after a delay from the leading edge 52 of a ½ cycle of the bi-directional square waveform. The time delay before the start of the high voltage pulse 50 is determined by the RC circuit of FIG. 3. By providing a high voltage pulse 50 during each ½ cycle of the bi-directional square waveform, the Lamp is ignited.
A drawback of the method and circuit described above is the inability of the circuit of FIG. 3 to provide a high voltage pulse 50 at the start of each ½ cycle of the bi-directional square waveform, while providing an efficient pulse starting circuit during normal operations of the lamp. Providing a high voltage pulse at the start of a ½ cycle of the bi-directional square waveform provides relatively more time for an electrode to heat before the ½ cycle of the bi-directional square waveform changes polarity. This increased temperature of the electrode will provide a reduction in sputtering.
The inefficiencies of the circuit of FIG. 3 are related to R1 40. Specifically, R1 40 must be decreased to a small value to enable this circuit to generate a high voltage pulse near the beginning of a ½ cycle of the bi-directional square waveform. By decreasing R1 40 to a small value, this pulse starting circuit will draw relatively more current and power during normal operation of the gas discharge lamp and consequently be less efficient.
Accordingly, an improved efficient pulse starting method and circuit are needed to start a gas discharge lamp.